The present invention concerns improvements in emission control, more especially, it concerns improvements in the control of regulated emissions from diesel engines.
Light duty diesels, for example those of up to about 2.5 liter capacity used in private cars and light vans, show much better fuel economy and hydrocarbon emissions than gasoline engines and are increasing in number. Emissions from diesel engines are now being regulated by legislation, and whilst hydrocarbon emissions do not present a problem in meeting emission limits, NOx is problematical. Since the exhaust gases from lean-burn engines such as diesels are high in oxygen content throughout the cycle of operation, it is more difficult to reduce NOx to N.sub.2. A contribution to this problem is the low exhaust gas temperature in comparison to gasoline-fuelled engines, which can also vary quite dramatically according to operating conditions. Thus, for a state of the art diesel exhaust catalyst system, 50% of hydrocarbons are oxidised at a temperature in the range 190 to 250.degree. C., and this inclines to 100% at higher temperatures. The conversion of NOx, however, shows very different characteristics, with a marked peak, creating a "window" of conversion at the 30 to 60% level, at a range of temperatures depending upon the catalyst system and engine system. This is illustrated in accompanying FIG. 1. As mentioned, the exhaust gas temperature will vary over a test cycle, so that the catalyst is seldom at a suitable temperature for optimum conversion. Over a test cycle therefore, the cumulative conversion efficiency for diesel NOx may only be about 10%. In fact, as the catalyst temperature drops below the peak conversion temperature for NOx, the temperature is also below "light off" for HC conversion. Accordingly, most of the unburnt hydrocarbons go out the tail pipe unreacted, along with unconverted NOx. It is to be noted that the exhaust temperature with heavy duty diesels is much higher and therefore these do not face quite the same problems. Nonetheless, the present invention offers improvements in emission control particularly during start-up of heavy duty diesel engines, and offers the potential to control emissions to meet expected future regulations.